The section De Commemorationibus in the Rubricae generales Missalis in later editions of the Missal of Pope Pius V begins by stating that "Commemorations occur at Mass as in the Office.
A Double or Semidouble Feast commemorated as a Simple in the Office is commemorated also at Mass, including Solemn Mass on Class II Double Feasts, but excluding Palm Sunday and the Vigil of Pentecost.
The complicated rules in their regard were given in the seventeen subsections of the section De Orationibus of the Rubricae generales Missalis.
By the decree Cum nostra hac aetate (De rubricis ad simpliciorem formam redigendis) of 23 March 1955[10] Pope Pius XII reduced the feasts previously of Simple rank to commemorations in the Office and Mass of the feast day or feria on which they occurred.
[12] The basis for some of the previous distinctions was removed by decreeing that feasts, except those of the first and second classes, would no longer, in line with the tradition of Jewish origin that counts sunset as the start of a new day, begin with First Vespers.
[13] Commemoration was always to be made of Sundays, First-Class Feasts, Ferias of Advent and Lent, the September Ember Days, and the Major Litanies.
[15] Five years later, the Code of Rubrics, which was composed by the same commission that had prepared the decree Cum nostra hac aetate, added little.
Only a few saints are classified in the General Roman Calendar as solemnities or feasts; the remainder are memorials, most of them optional.
[17] The Liturgy of the Hours as revised by Pope Paul VI and promulgated in 1970[18] prescribes that on the days when in Mass the collect is the only part of a memorial that may be used one may: This optional arrangement on such days in Mass and in the Liturgy of the Hours has obvious similarities with the earlier arrangements concerning commemorations.